Stop Wasting Hours on SvelteKit Auth - Let AI Fix These 7 Painful Issues

# Get the LLM-optimized docs
curl -o svelte-docs.txt https://svelte-llm-docs.khromov.se/svelte-5-sveltekit-distilled.txt

Using the SvelteKit documentation you have, create a complete Auth.js setup with:
- Google OAuth and GitHub OAuth
- Magic link authentication
- Proper TypeScript types
- CSRF protection that works in production
- Session handling that persists on Safari
- Error boundaries for auth failures

Include all the edge case handling and production considerations.

// src/hooks.server.ts
import { SvelteKitAuth } from '@auth/sveltekit';
import Google from '@auth/sveltekit/providers/google';
import GitHub from '@auth/sveltekit/providers/github';
import Nodemailer from '@auth/sveltekit/providers/nodemailer';
import { PrismaAdapter } from '@auth/prisma-adapter';
import { prisma } from '$lib/prisma';
import { 
  GOOGLE_CLIENT_ID,
  GOOGLE_CLIENT_SECRET,
  GITHUB_ID,
  GITHUB_SECRET,
  AUTH_SECRET,
  SMTP_HOST,
  SMTP_USER,
  SMTP_PASSWORD
} from '$env/static/private';

export const { handle, signIn, signOut } = SvelteKitAuth({
  adapter: PrismaAdapter(prisma),
  providers: [
    Google({
      clientId: GOOGLE_CLIENT_ID,
      clientSecret: GOOGLE_CLIENT_SECRET,
    }),
    GitHub({
      clientId: GITHUB_ID,
      clientSecret: GITHUB_SECRET,
    }),
    Nodemailer({
      server: {
        host: SMTP_HOST,
        port: 587,
        auth: {
          user: SMTP_USER,
          pass: SMTP_PASSWORD,
        },
      },
      from: 'noreply@yourdomain.com',
    }),
  ],
  secret: AUTH_SECRET,
  trustHost: true, // Critical for production deployment
  pages: {
    signIn: '/auth/signin',
    error: '/auth/error',
  },
  callbacks: {
    async session({ session, token }) {
      // Ensure session data persists correctly
      session.user.id = token.sub;
      return session;
    },
    async jwt({ token, user }) {
      if (user) {
        token.sub = user.id;
      }
      return token;
    },
  },
  events: {
    async signIn({ user, account, profile }) {
      console.log('Sign in event:', { user: user.email, provider: account?.provider });
    },
    async signOut({ session }) {
      console.log('Sign out event:', { user: session?.user?.email });
    },
  },
});

I'm getting "Cross-site POST form submissions are forbidden" in my SvelteKit Auth.js setup. 

Here's my current setup:
[paste your hooks.server.ts]
[paste your environment variables (without secrets)]
[paste the browser network tab showing the failed request]

The error happens when: [describe exactly when it occurs]
My deployment: [Vercel/Netlify/etc]
Browser: [Chrome/Safari/etc]

What's the root cause and how do I fix it?

// The fix is usually one of these:
export const { handle, signIn, signOut } = SvelteKitAuth({
  // ... your config
  trustHost: true, // This fixes most CSRF issues
  
  // OR if you need more control:
  callbacks: {
    async redirect({ url, baseUrl }) {
      // Ensures redirects stay on your domain
      if (url.startsWith("/")) return `${baseUrl}${url}`;
      if (new URL(url).origin === baseUrl) return url;
      return baseUrl;
    },
  },
});

My SvelteKit auth sessions aren't persisting on page refresh. Here's my setup:

[paste your +layout.server.ts]
[paste your +layout.svelte]
[paste any auth-related load functions]

Users can sign in but get logged out on refresh. This happens more on Safari than Chrome.
What pattern am I missing?

// WRONG - This was causing my persistence issues
// src/routes/+layout.svelte
<script>
  import { page } from '$app/stores';
  let session = $page.data.session; // Undefined on refresh!
</script>

// RIGHT - AI suggested this pattern
// src/routes/+layout.server.ts
import type { LayoutServerLoad } from './$types';

export const load: LayoutServerLoad = async (event) => {
  const session = await event.locals.auth();
  
  return {
    session, // Always available, even on refresh
  };
};

<!-- src/routes/+layout.svelte -->
<script>
  import type { LayoutData } from './$types';
  
  export let data: LayoutData;
  $: ({ session } = data); // Now this persists!
</script>

{#if session}
  <p>Welcome, {session.user?.name}!</p>
  <button onclick={() => signOut()}>Sign out</button>
{:else}
  <button onclick={() => signIn()}>Sign in</button>
{/if}

My SvelteKit Auth.js setup fails on mobile Safari but works on desktop. Issues:
- Sign-in button doesn't respond
- OAuth redirects show "This form isn't secure"
- Sessions don't persist on mobile

What Safari-specific issues should I check for in Auth.js?

// src/hooks.server.ts - Safari-friendly configuration
export const { handle, signIn, signOut } = SvelteKitAuth({
  // ... other config
  
  cookies: {
    sessionToken: {
      name: 'authjs.session-token',
      options: {
        httpOnly: true,
        sameSite: 'lax', // Critical for Safari
        path: '/',
        secure: true, // Must be true in production
      },
    },
    callbackUrl: {
      name: 'authjs.callback-url',
      options: {
        sameSite: 'lax', // Safari requires lax, not strict
        path: '/',
        secure: true,
      },
    },
  },
  
  useSecureCookies: true, // Essential for Safari
});

<!-- src/routes/auth/signin/+page.svelte -->
<!-- Safari-friendly sign-in buttons -->
<script>
  import { signIn } from '@auth/sveltekit/client';
  
  // AI suggested this pattern for Safari compatibility
  async function handleSignIn(provider: string) {
    try {
      await signIn(provider, { 
        callbackUrl: '/',
        redirect: true 
      });
    } catch (error) {
      console.error('Sign in error:', error);
      // Fallback for Safari
      window.location.href = `/auth/signin/${provider}`;
    }
  }
</script>

<!-- Use buttons, not links, for better Safari support -->
<button onclick={() => handleSignIn('google')}>
  Sign in with Google
</button>
<button onclick={() => handleSignIn('github')}>
  Sign in with GitHub
</button>

Create error boundaries and user-friendly error handling for every possible SvelteKit Auth.js failure:
- OAuth provider failures
- Network timeouts
- Invalid sessions
- CSRF errors
- Database connection issues

Show me the complete error handling strategy.

// src/routes/auth/error/+page.server.ts
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async ({ url, locals }) => {
  const error = url.searchParams.get('error');
  const code = url.searchParams.get('code');
  
  let errorMessage = 'An authentication error occurred.';
  let suggestion = 'Please try signing in again.';
  
  switch (error) {
    case 'Configuration':
      errorMessage = 'Authentication is not properly configured.';
      suggestion = 'Please contact support.';
      break;
    case 'AccessDenied':
      errorMessage = 'Access was denied.';
      suggestion = 'You may have cancelled the sign-in process.';
      break;
    case 'Verification':
      errorMessage = 'Unable to verify your identity.';
      suggestion = 'The sign-in link may have expired.';
      break;
    case 'OAuthCallback':
      errorMessage = 'OAuth provider error.';
      suggestion = 'Please try a different sign-in method.';
      break;
    default:
      if (code === 'credentials') {
        errorMessage = 'Invalid credentials.';
        suggestion = 'Please check your email and password.';
      }
  }
  
  return {
    error: errorMessage,
    suggestion,
    code
  };
};

<!-- src/routes/auth/error/+page.svelte -->
<script>
  import type { PageData } from './$types';
  export let data: PageData;
</script>

<div class="auth-error">
  <h1>Authentication Error</h1>
  <p class="error-message">{data.error}</p>
  <p class="error-suggestion">{data.suggestion}</p>
  
  <div class="error-actions">
    <a href="/auth/signin" class="button">Try Again</a>
    <a href="/" class="button-secondary">Go Home</a>
  </div>
  
  {#if data.code}
    <details class="error-details">
      <summary>Technical Details</summary>
      <p>Error Code: {data.code}</p>
      <p>If this error persists, please contact support with this code.</p>
    </details>
  {/if}
</div>

<style>
  .auth-error {
    max-width: 400px;
    margin: 2rem auto;
    padding: 2rem;
    text-align: center;
  }
  
  .error-message {
    font-size: 1.1rem;
    margin: 1rem 0;
    color: #dc2626;
  }
  
  .error-suggestion {
    color: #6b7280;
    margin: 1rem 0;
  }
  
  .error-actions {
    margin: 2rem 0;
  }
  
  .button {
    display: inline-block;
    background: #3b82f6;
    color: white;
    padding: 0.75rem 1.5rem;
    border-radius: 0.375rem;
    text-decoration: none;
    margin: 0 0.5rem;
  }
  
  .button-secondary {
    display: inline-block;
    background: #6b7280;
    color: white;
    padding: 0.75rem 1.5rem;
    border-radius: 0.375rem;
    text-decoration: none;
    margin: 0 0.5rem;
  }
  
  .error-details {
    margin-top: 2rem;
    text-align: left;
    font-size: 0.875rem;
    color: #6b7280;
  }
</style>